It has been proposed in the prior art that the pulping of cellulosic chips in a digester, employing the kraft process, may be enhanced through "sulfide profiling", a procedure in which the white liquor containing the pulping chemicals is introduced to the digester in the form of a plurality of separate inlet streams of white liquor. The sulfidity of each inlet stream is adjusted to a predetermined value and each inlet stream is introduced to the digester at a location at which the white liquor of adjusted sulfidity is optimally efficient in digestion of the wood. The prior art suggests that each inlet stream of white liquor be "made up" by mixing selected amounts of sodium hydroxide and sodium sulfide compound(s) with a solvent, such as water. That is, one would use raw chemicals to make up each inlet stream. The expense and time required to thusly make up, store and dispense separate inlet streams for a digester makes it impractical to employ sulfide profiling as proposed in the prior art. Moreover, the addition of raw chemicals upsets the "chemical balance" of the pulping and/or recovery operations, and to maintain operations, purge points would be needed.
In the kraft cellulosic pulping process, the black liquor from the digester contains considerable chemical values (at times referred to as "process salts") which desirably are recovered and reused within the papermaking process. Process salts contained in the black liquor from a kraft process digester include sodium sulfate and sodium carbonate. It is taught in the prior art that black liquor from a kraft pulp digester may be introduced, along with an amphoretic oxide such as TiO.sub.2, and air into a fluidized bed reactor (FBR) to produce a cake which contains the process salts from the black liquor. Zou et al., in their publication entitled: "Kraft Black Liquor Combustion and Direct Causticization with Titanium Dioxide", TAPPI Proceedings, 1991 Pulping Conference, pp. 299-308, which publication is incorporated herein by reference, provide a discussion of this prior art process for reacting kraft black liquor with TiO.sub.2 in a fluidized bed, including the chemistry involved. Generally stated, in this prior art process direct causticizing (at times referred to as "autocausticizing") of the process salts takes place in the FBR as a reaction between the sodium carbonate and the titanium dioxide to form an intermediate sodium titanate. Further, within the FBR the sulfur compounds in the black liquor are oxidized to sulfate. The pertinent result is the formation in the FBR of a cake which comprises a mixture of sodium sulfate and sodium titanate. In the prior art, this cake, preferably while in the solid state, is treated with a reductant to convert the sulfates in the cake to sulfides. The inorganic chemical values in the reduced cake thus include sodium sulfide and sodium titanate (4Na.sub.2 O.5TiO.sub.2). The sodium sulfide is in the form of solid particulates that are integrally mixed with the solid sodium titanate. The sodium in the sodium titanate is chemically complexed and may be recovered from the reduced salts only through a chemical reaction.
In the prior art, the reduced "fresh salts" are leached with water to extract and solubilize the sodium sulfide from the solid salts and, through hydrolysis, to produce sodium hydroxide and Na.sub.2 O.3TiO.sub.2. The sodium hydroxide enters into solution and the Na.sub.2 O.3TiO.sub.2 is a solid residue which can be dewatered and recycled to the black liquor entering the FBR. The filtrate from the leaching procedure of the prior art black liquor recovery process is collected as a single stream of white liquor which contains sodium hydroxide and sodium sulfide. The sulfidity of this single stream is commonly about 30%.
It will be apparent that in this prior art, there is no possibility of using the single filtrate stream containing sodium hydroxide and sodium sulfide in a sulfidity-profiling pulp digestion process which requires multiple white liquor infeed streams, each of which has a sulfidity value which is different from the sulfidity value of each other infeed stream. Further, in the prior art, the recovery process provides only for an outlet stream which contains both sodium sulfide and sodium hydroxide so that the outlet stream is useful only for recycling to the digester.
It is an object of the present invention to provide a method for recovering sodium sulfide values and sodium hydroxide values from a kraft black liquor.
It is another object of the present invention to provide a method for recovering chemical values from kraft black liquor in multiple streams containing recovered chemical values which are useful as inlet streams to one or more of the operations of a papermaking process.
It is another object of the present invention to provide a method for developing a plurality of inlet streams of reaction chemicals for a cellulosic pulp digester operating in a sulfide profiling mode.
It is another object of the present invention to provide a method for treating a kraft black liquor to develop a plurality of streams containing chemical values recovered from the black liquor, each stream having different chemical values and/or different quantities of one or more of the chemical values in the black liquor.